Exposure to free radicals cause a distinct sequence of changes in the cardiocyte action potential. The increased plateau potential, action potential duration and subsequent after-depolarizations suggest an influx of calcium into the cell. In this proposal, isolated canine cardiocytes are exposed to the free radical, H2O2, and the action potential monitored as a means of assessing the free radical-induced injury to the membrane. Parallel superfusion experiments provide cells with the free radical-induced injury for a biochemical assessment of membrane damage as well. The possible involvement of a calcium channel in the altered action potentials is evaluated using calcium channel antagonists and different concentrations of extracellular calcium. These agents are to be superfused over cardiocytes during exposure to free radicals. The action potentials are monitored and the time course of free radical-induced injury determined. Protection by calcium channel antagonists or altered time course of free radical injury would suggest a direct effect of free radicals on the calcium channel. Since different calcium antagonists may bind to different sites on the calcium channel, the results may also help to localize a site of free radical-induced injury. In addition, adrenergic agents will be used, since activation of adrenergic receptors also effects calcium channels. Free radical-induced changes in the state of adrenergic receptors could open calcium channels and result in calcium overload. The assessment of lipid peroxidation during exposure of cardiocytes to free radicals and pharmacologic agents will suggest whether the extent of free radical-induced injury is changed (i.e., the pharmacologic agents act as scavenbers), or if the influx of calcium into the cell is altered. The results from this proposal may suggest a mechanism by which free radicals alter calcium permeability in the isolated cardiocyte. Free radical injury has been implicated in the myocardial injury due to diverse conditions from the oxygen paradox to adriamycin toxicity and aging. A more complete understanding of the mechanisms of free radical-induced damage to the membrane could suggest new therapeutic approaches in a clinical setting.